The most common and most likely answer to this crossword clue is the 5 plus 3 letter word STUFF YOU
I've seen this crossword clue in Independent
S
T
U
F
F
Y
O
U
Buddy explains!
The straight part
We know that "get lost!" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case get lost! = stuff you.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.